The conventional process for manufacturing alkylene glycol from an alkene, oxygen (or air) and water is described in FIG. 1 in the context of manufacturing ethylene glycol. The first stage is the gas phase oxidation of ethylene over a heterogeneous catalyst to produce ethylene oxide (EO). In order to achieve the desired reaction selectivity and reduce the hazards associated with EO manufacturing, the EO concentration is limited in the outlet reaction gas stream. Only partial oxidation of ethylene per pass occurs and as a consequence, the reactor outlet gas contains not only EO, but also unreacted ethylene and oxygen and, among other things, byproduct carbon dioxide, water and parts per million levels of aldehydes, such as acetaldehyde. Ballast gas, typically methane or nitrogen, is also present in the EO reactor outlet.
Upon exiting the EO reactor, the gaseous stream is typically cooled in one or more heat exchangers (not shown) to transfer the heat contained in the EO reactor gas to other process streams to reduce the energy requirements in the overall process. Although the energy integration is advantageous, the extra residence time required to pass through the heat exchangers can increase the amount of unintended side reactions that occur in the EO reactor outlet gas due to the high reactivity of ethylene oxide thus forming increased levels of impurities such as aldehydic compounds, chloride-containing species and oxygenated species. After being cooled, the gas stream is passed to an EO absorption unit. In the EO absorber, a large excess of water, typically 8:1 weight ratio of water to ethylene oxide or higher, is used to absorb the EO in the EO reactor outlet gas. The unreacted ethylene, by-product carbon dioxide and other unabsorbed compounds are passed to a separation unit in which a portion of the carbon dioxide is separated from the EO-lean EO reactor outlet gas, and the remaining gas is recycled back to the EO reactor. This return of the unreacted ethylene, oxygen, ballast gas and the remaining other components to the EO reactor is known as the recycle gas loop.
EO and water from the EO absorber are typically pre-heated in an exchanger and passed as a single stream to an EO stripping unit in which EO is separated from the water by vaporization. The amount of energy required to strip EO from the water is high since such a large excess of water is required in the absorption step. The tails stream from the stripping unit, containing the vast majority of the water is cooled and recycled back to the EO absorption unit. The stripped EO is partially condensed and/or reabsorbed before some of the EO is refined by passing through one or more distillation columns, and then collected for storage, shipping or use.
The second stage of the process is the liquid phase hydrolysis of EO to ethylene glycol. In the conventional manufacturing of mono-ethylene glycol (MEG), refined EO is diluted with a 10-15 fold weight-by-weight (w/w) excess of water (H2O) for a H2O:EO molar ratio of 24:1 to 37:1, and then thermally hydrolyzed. In some instances, a solid catalyst may be used to improve the selectivity to the preferred product of MEG. The conventional uncatalyzed liquid-phase process produces an assortment of glycol products including the desired MEG (e.g., 90-91 wt %), di-ethylene glycol (DEG) (e.g., 8-9 wt %) and 1 wt % or less of other higher molecular weight glycols, although the exact distribution depends largely upon the ratio of water to ethylene oxide in the dilution step. MEG is further refined by first removing the large excess of water in multiple evaporators and then distilled in several columns under reduced pressure. Water removed from the glycol product is recycled back to the beginning of the process for admixture with EO before or at the time EO is fed to the glycol reactor. The large excess of water used in the liquid phase hydrolysis process requires a large amount of energy to drive the glycol/water separation.
While the liquid-phase EO reaction to MEG and other EO derivatives has been extensively studied, the gas-phase EO hydrolysis reaction has received relatively little study. In the mid 1980's and early 1990's, Union Carbide Corporation researchers investigated the gas phase hydrolysis of EO to MEG, e.g., U.S. Pat. Nos. 4,701,571 and 5,260,495. While several compounds demonstrated a high degree of selectivity to MEG at a hydrolysis ratio of 1 w/w or lower, the catalyst activities and catalyst lifetimes limited the commercial appeal of the technology at that time. Therefore new gas phase hydrolysis catalysts that provide high MEG selectivity with improved catalyst activity, catalyst lifetime, stability and operation versus the prior disclosed catalysts are desired. Additionally, a gas phase hydrolysis process that provides significant economic benefits and addresses the drawbacks with conventional EO/EG processes is needed, such as energy requirements and impurity generation.